Interventions designed to reduce STI/HIV risk have mostly failed to consider clients of FSWs, who may act as a 'bridge' to the general population. Our recent survey of U.S and Mexican clients seeking FSWs in Tijuana revealed a startling HIV prevalence rate of 4.1%. Based on qualitative and quantitative interviews with clients from Tijuana Mexico and the U.S. we developed a theory-based intervention (Hombre Seguro) that uses Motivational Interviewing, active participation, and problem solving to increase clients' use of condoms with FSWs. We now propose an RCT to test the efficacy of this one- hour intervention in Tijuana which will enroll 400 HIV-negative men (200 residents of San Diego County; 200 Tijuana residents) who report having unprotected sex with FSWs in the past 4 months. Our theory-based participatory intervention will be evaluated in comparison with a time and information equivalent didactic control condition. Participants will be recruited in Tijuana where they will undergo a baseline CAPI-administered interview, STI testing (HIV, syphilis, gonorrhea, Chlamydia), and intervention counseling with follow-up interviews at 4, 8, and 12 months post-baseline conducted in either Tijuana or San Diego. Our study aims are to: 1) evaluate the efficacy of Hombre Seguro to increase condom use of clients with FSWs; 2) determine if the intervention is as efficacious among U.S. versus Mexican clients; 3) determine the extent to which theoretically-based components of our intervention (e.g., self-efficacy) represent underlying mechanisms of change in primary outcomes (e.g., lower HIV/STI incidence); and 4) explore subgroup differences in efficacy of the intervention based on background characteristics (e.g., age), contextual factors (e.g., substance use before/during sex), psychosexual factors (e.g., social-sexual effectiveness), and psychosocial factors (e.g., social network influence). Our primary analytic approach will utilize a generalized linear mixed models (GLMM) approach to repeated measures analysis. This bi-national study will contribute significantly to HIV prevention research by addressing the role of male clients in the escalating HIV epidemics in Tijuana and San Diego, and could have application to other settings where HIV prevalence is high among FSWs and their clients.